2 coax in series

 Does the amp have a high pass filter switch?  If so, you could run the coaxials through the high pass on the amp set at about 80 Hz.  That will give you the low cut-off.  Then they'll operate at 80 Hz and above.  The amp should work at 2 ohm stereo...check and see if it is spec'd to run below 4 ohm.  If it will, you can parallel the rights and lefts for a 2 ohm stereo load.  No fade control to attenuate the rears, though.  Your power would be meager at an 8 ohm load on that amp, running four speakers.

Best thing, leave the coaxials intact.  Use your amp for the fronts, head unit for the rears.  With an eye toward the future, as suggested above, search for a four channel amp to power the speakers.  If the amp you have now is bridgeable, it would be suitable to power a sub.  You want to maintain a 4 ohm load on the speakers in the car for sound quality.  8 ohms would be good, but with a more powerful and more versatile amp...and higher quality component speakers....else you won't be able to appreciate the difference in sound quality.

To answer your question, a crossover is designed so that (to put it simply) only one speaker is "connected" to the amp at a time.  So the amp will only ever see the impedence load from one speaker.

Now, why do you want to put multiple speakers in each door?  What do you hope to gain?

Thanks for the replys.  To answer a few questions, the amp is what people are calling "old school" now.  Not bridgeable and doesn't have any support to suggest that less than 4 ohms is ok.  As for the speakers, I have two really nice sets of coaxials from some old installs, the amp is an old kenwood from an old install also.  The speakers are memphis and jl audios with some pretty decent specs.  I am sort of building up to a nice system so wanted to use what I have for now.  I can split the memphis coaxials since the tweeters are wired with separate wires from a parallel junction at the rear of the speakers unlike some that wire through the midrange cone.  I think for simplicity, I will wire the fronts to the old amp and the rears to the deck.  That still leaves the question of what are the resistance of each component and how to cross them over so as to end up with one 4 ohm load.  Has anyone tried this? 

Thanks for the replies.  Based on what I have read, if a typical coax is powered from 80 hz and above and only has the capacitor in line on the tweeter portion, there is likely a duplication of sound and an unnatural decibel increase over the ranged that they overlap.  I guess this is the reason for components.  That said, my purpose in looking into splitting the coaxials and adding some nice crossovers is to creat a more linear response from the tweeter/midwoofer pair.  I got some feedback from Memphis.   They said that the tweeter is somewhat weak on their paired coax and relies heavily on the midwoofer.   Both have 3.6 ohms impedence but the tweeter doesn't pick up until about 7khz.  From what I have seen, most band pass crossovers cross at around 5k on the high end.  

I am still not convinced to go ahead and separate them, but it is tempting.  If I did, I am guessing that a nice band pass from 80 to 7k  and then a high pass above 7k would work well with these speakers.  Any thoughts from those of you who have more experience than me with this sort of thing?